LONDON — Toys “R” Us said on Wednesday that it would close all of its stores in Britain after failing to find a buyer, another ominous sign for a once-dominant toy retailer that has struggled under pressure from Amazon and other online sellers. The company, which filed for bankruptcy in the United States last year, said it would close its remaining 75 stores in Britain in addition to the 25 it has already shut. Toys “R” Us has also closed more than 100 stores in the United States in hopes of...